# Difference between subsetting methods

**URL:** <https://discourse.julialang.org/t/difference-between-subsetting-methods/73951>\
**Category:** General Usage\
**Tags:** dataframes\
**Created:** [January 3, 2022, 1:29am UTC](https://discourse.julialang.org/t/difference-between-subsetting-methods/73951 "2022-01-03T01:29:08Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![aachener](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/aachener/32/18112_2.png) [@aachener](https://discourse.julialang.org/u/aachener)\
**Post date:** [January 3, 2022, 1:29am UTC](https://discourse.julialang.org/t/difference-between-subsetting-methods/73951/1 "2022-01-03T01:29:09Z")

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Hi, guys,

I’ve got a basic question about subsetting of DFs. As shown below, what the difference between dd.y[1:2] and dd[1:2, :y]? My guess is that the former is in-place and the latter is copy of the selection. But they all do in-place modifications as in the code. So I am confused.

Maybe I missed something in the Doc, please direct me to the right place of the doc if I did. Thanks!

```julia
dd = DataFrame(x = [1, 2, 3, 4], y = rand(1:10, 4))
dd.y
dd[!, :y]
dd.y === dd[!, :y] #true
dd.y[1:2] === dd[1:2, :y] #false

# they all do in-place modifications, but they are not the same?
dd.y[1:2] = [0, 0]
dd
dd[1:2, :y] = [1, 1]
dd

```

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<div class="post-metadata">

**Author:** ![jd-foster](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/jd-foster/32/35824_2.png) [@jd-foster](https://discourse.julialang.org/u/jd-foster)\
**Post date:** [January 3, 2022, 8:59am UTC](https://discourse.julialang.org/t/difference-between-subsetting-methods/73951/2 "2022-01-03T08:59:07Z")

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You may or may not have seen this page in the docs: [Indexing · DataFrames.jl](https://dataframes.juliadata.org/stable/lib/indexing/)

As you observe, `dd.y === dd[!, :y]`, and in fact, that is precisely the way that `dd.y` is defined.  
It is also true that `dd.y[1:2] == dd[1:2, :y]` (here with `==` not `===`).  
But the observation

```julia
dd.y[1:2] === dd[1:2, :y] #false

```

is based in the Julia language, not DataFrames in particular:

```julia
 x = [1,2,3,4];
 x[1:2] === x[1:2] # false

```

There is a tiny difference: `dd.y` is returning a reference to the `Vector` that holds the column `:y` from within the dataframe `dd` then providing the indexed elements at `1:2` with the `getindex` method of the Julia `Vector` type in the Base module, while `dd[1:2, :y] ` is doing all that within an internal DataFrames function.

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<div class="post-metadata">

**Author:** ![aachener](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/aachener/32/18112_2.png) [@aachener](https://discourse.julialang.org/u/aachener)\
**Post date:** [January 3, 2022, 9:09am UTC](https://discourse.julialang.org/t/difference-between-subsetting-methods/73951/3 "2022-01-03T09:09:52Z")

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Many thanks! That’s pretty clear. I will go through the indexing part of doc.